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Understanding the complex formation of metal ions in room-temperature ionic liquids (ILs) is essential for the application of ILs in solvent extraction. Nevertheless, the research on metal complex formation in ILs lags behind other applications. The complex formation equilibria may be influenced by specific interactions among the metal ion, ligand molecule, and IL cation and anion. In the present investigation, the complex formation of Ni2+with ethanol (EtOH) and methanol (MeOH) molecules in ILs, 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([CNmim][TFSA], where Nrepresents the alkyl chain lengths of 2 and 8) was discussed in terms of the microscopic interactions among alcohol molecules, [CNmim]+and [TFSA]−, and the mesoscopic mixing states of alcohols in [CNmim][TFSA], with N= 2–12. The microscopic interaction of alcohol molecules with the imidazolium ring H atoms in the ILs was evaluated by using ATR–IR and 1H and 13C NMR spectroscopies. The self-hydrogen bonding of alcohol molecules was clarified from the O–H stretching vibration of alcohol molecules. MeOH molecules can be more strongly hydrogen-bonded with themselves than EtOH molecules due to the less steric hindrance and the weaker dispersion force of MeOH with the IL cation’s alkyl chain. In fact, small-angle neutron scattering (SANS) experiments revealed the more heterogeneous mixing of MeOH with the ILs by the self-hydrogen bonding among MeOH molecules than EtOH. The longer the IL cation’s alkyl chain, the more the MeOH clusters significantly form. In contrast, the formation of EtOH clusters becomes weaker with elongating the alkyl chain. Ultraviolet (UV)–visible spectroscopic measurements on Ni2+–[CNmim][TFSA]–alcohol solutions with N= 2 and 8 revealed that di-, tetra-, and hexa-alcohol–Ni2+complexes are formed in both the ILs. With N= 2, the stabilities of Ni2+–EtOH and Ni2+–MeOH complexes are comparable in the IL. However, with N= 8, the complexes are more stable in the EtOH solutions than in the MeOH solutions. This is because the less heterogeneous mixing of EtOH molecules with the IL results in the larger enthalpic contribution in the complex formation, as shown by the thermodynamic parameters estimated by the van’t Hoff plots on the stability constants at several temperatures.

Because hydroxypropyl cellulose (HpC) is a popular polymeric material that forms a liquid crystalline phase in solutions with various kinds of solvents, including water, it is commonly thought that HpC has a typical rod-like structure in solution. In this study, the structures of commercial HpC samples in aqueous solution with average molar substitution numbers (MS) ranging from 3.6 to 3.9 and weight-average molar masses (Mw) ranging from 36 to 740 kg mol–1were investigated in detail. We first used multiple techniques, including standard static and dynamic light scattering (SLS and DLS), neutron and X-ray scattering experiments, and viscometric measurements, to obtain clear evidence of rod-like structures quantitatively. The dependence of excess scattering intensities for HpC samples under dilute conditions on the magnitude of the scattering vector over a wide range from 8.9 × 10–3to 3.0 × 10 nm–1was reasonably described by the form factor of rod particles with length (L) and diameter (d). Although the determined Lvalue was close to the contour length (lc) calculated from the Mwvalues in the lower Mwrange, Lbecame obviously less than lcwith increasing Mw. The radius of gyration (Rg) determined via SLS measurements was proportional to Lby a factor of approximately 3.5 ∼ √12 over the Mwrange examined. These observations revealed that the conformation of HpC molecules changes from an elongated single chain to a certain folded structure, maintaining the shape of the rod-shaped particles. Moreover, the Mwdependencies of the intrinsic viscosities and translational diffusion coefficients of the HpC samples resulting from DLS measurements were reasonably described with a theoretical rod-like particle model, assuming that Land dare identical to those resulting from the scattering behaviors.